This is the bringing-coals-to-Newcastle rule: If a sector is already well represented in your portfolio, why buy more of it? If you're going to make use of a sector fund, it should add something your portfolio lacks, or it should increase your exposure to a sector that is underrepresented in your portfolio.

To determine whether you should buy more funds in a particular sector, you need to know and monitor your portfolio's sector weightings. Vanguard Total Stock Market, for example, owns only 3.18% in utilities as of March 31. More conservative investors who like the dividends that utilities stocks often pay out might want to right bump up their exposure with a utilities sector fund.